1. Field of the Invention
The present invention relates to a communication system, more particularly, to a receiving apparatus of a spread spectrum signal.
2. Description of Related Art
For a multiple access system in which a plurality of stations communicate with each other using an allocated frequency band, there are proposed various communication systems such as a frequency division multiple access (FDMA) system, a time division multiple access (TDMA) system and a code division multiple access (CDMA) system. In most of these systems, a service area is divided into a plurality of small cells and a base station is located in each of the plurality of cells. A subscriber communicates with another subscriber via the base station.
Among these multiple access systems, because the CDMA system does not require burst synchronization, it is suitable as a communication system composed of many subscriber equipments. Also, the CDMA system has the advantage that it is not affected by interference and noise. As a result, the CDMA system is receiving much attention. The CDMA system using the spread spectrum communication system is one multiple access system in which different spreading code sequences are assigned to users and spreading modulation is performed using the spreading code sequences, respectively. As a result, the same frequency band can be used by many users in a cell.
As already known, it is assumed in the spread spectrum communication system that the spreading code sequence used on a transmission side is synchronous with used on a reception side when a received signal is despread. Therefore, for instance, in a case where the phase of spreading code sequence is shifted over one chip due to influence of change of a delay amount on a transmission path which depends on multiple paths or the like, it becomes difficult to accurately demodulate data. Hence, in the spread spectrum communication system, synchronization confinement (initial synchronization) and synchronization tracking (synchronization holding) are absolutely necessary. Normally, the synchronization confinement confines the difference in phase between a spreading code sequence on the transmission side and a spreading code sequence on the reception side into a sufficiently small range (normally below 1/2 chip). The synchronization tracking always keeps a synchronization position captured once with the precision below 1/2 chip such that the captured synchronization position is not lost because of influence of noise and modulation. For this reason, the synchronous control and stabilization control of a clock signal used in a receiving apparatus are important.
An example of such a conventional spread spectrum communication system will be described with reference to FIGS. 1 and 2A to 2C. FIG. 1 is a block diagram of the structure of a conventional spread spectrum signal receiving apparatus. FIGS. 2A to 2C are diagrams to explain the operation of a delay locked loop (DLL) circuit as a conventional frequency synchronizing circuit. In the receiving apparatus, the same spreading code sequence(a PN code sequence) as multiplied in the transmitting apparatus is generated by a spreading code generator 123 (the spreading code sequence used in the demodulation is referred to as "PN(0)" hereinafter). The generated spreading code sequence is multiplied in multipliers 126.sub.3 and 126.sub.4 for despreading. Thus, despreading demodulation is performed.
However, in this case, it is necessary to synchronize PN(0) with the spreading code sequence multiplied in the transmitting apparatus. For this reason, quasi-synchronization detection is performed to the received signal using a quadrature demodulator 102 composed of a shifter 122 and a local oscillator 121 generating a signal having the frequency approximately equal to that used in the transmitting apparatus to obtain an in-phase component and quadrature component (to be referred to as "I component and Q component" hereinafter ). The spreading code generator 123 independently generates, for each of the obtained I and Q components, a spreading code sequence having a phase slightly proceeding than the spreading code sequence used for despreading demodulation (typically, 1/2 chip) (to be referred to as "PN(+)" hereinafter) and a spreading code sequence having a phase delayed to the same extent (to be referred to as "PN(-)" hereinafter).
The received signal is multiplied by the respective spreading code sequences in multipliers 126.sub.5 and 126.sub.6 for despreading. High frequency components are removed from the multiplied signals by low pass filters 127.sub.2 and 127.sub.3 (to be referred to as "LPFs" hereinafter) for smoothing. As a result, despreading demodulation outputs of the I and Q components are obtained. A synthesis correlation signal of the spreading demodulation outputs of the I and Q components is inputted to a frequency control signal calculating circuit 125 for comparing the phases of spreading code sequences.
A frequency control signal is calculated for a voltage controlled oscillator (VCO) 124 and frequency synchronization is controlled in the spreading code generator 123 and the local oscillator 121 which are driven in accordance with a clock signal generated by the VCO 124, such that frequency synchronization with a carrier of the received signal is established and frequency synchronization with a local signal is established.
In this case, the I and Q component signals inputted to the frequency control signal calculating circuit 125 have the correlation output characteristics J and K shown in FIGS. 2A and 2B as the despreading correlation outputs depending on the received signal, PN(+) and PN(-). When these correlation outputs are added or subtracted by an adder 128, a synthesis correlation output characteristic L is obtained as the synthesis correlation output of J and K for each of the I and Q components, as shown in FIG. 2C. The frequency control signal to the VCO 124 which is used for the spreading code generator 123 is determined from the synthesis correlation output characteristics L for the I and Q components. In actuality, PN(0) of the spreading code generator 123 tracks the spreading code sequence of the received signal multiplied in the transmitting apparatus such that synchronization with the spreading code sequence is established. As a result, a middle point of the maximum output value and minimum output value of the synthesis correlation output characteristic L is set to "0". That is, the control is performed to stably generate the spreading code sequence PN(0) which is used for demodulation, at a point L0 of FIG. 2C.
As examples of such a conventional technique, there are the systems disclosed in Japanese Laid Open Patent Disclosures (JP-A-Hei3-101534. JP-A-Hei5-308345 and JP-A-Hei2-92035).
However, in the above-mentioned conventional technique, the PN(+) having phase proceeded and the PN(-) having phase delayed need to be always subjected to the despreading process in addition to the spreading code sequence PN(0) used to demodulate information from the signal actually received. For this reason, the spreading code generators and processing sections need to be provided for the PN(-) and PN(+) so that the circuit size increases.